Okay, we’re taking on something pretty big here – the genesis of the human heart! It’s a big topic in that it is the first major system formed and we often use it as the basis of determining when life begins.

The cardiovascular system is derived from the splanchnic mesoderm, the lateral and paraxial mesoderm, the pharyngeal mesoderm and neural crest cells. The heart itself has three layers, the endocardium, myocardium and the visceral pericardium. At the very beginning, it really looks a lot more like a tube than any kind of heart we would think of. Remember earlier when we talked about the two cardiac tubes fusing? Still kind of looks like a tube, right?

Well, take that very first tube, stretch it out a bit and start to put some bulbs in it. It is dilating and constricting at certain places, so you can think of it more vase like. There are five dilations, starting from top to bottom or which is also reverse order of blood flow and including what it will become in an adult: Trunchus asteriosus (aorta, pulmonary trunk); Bulbus cordis (Smooth part of right ventricle via the conus arteriosus, smooth part of left ventricle via the aortic vestibule); Primitive ventricle (Trabeculated part of right ventricle, trabeculated part of left ventricle, aka the real meaty tough muscle parts); Primitive atrium (Trabeculated part of right atrium, trabeculated part of left atrium); Sinus venosus (Smooth part of right atrium via sinus venarum, coronary sinus and oblique vein of left atirum).

The truncus arteriosus, the top dilation, connects to the aorta sac, so the blood is going to flow out at that point. The sinus venosus connects to the umbilical, vitelline and common cardinal veins from the chorion, so that is where blood is going to come in.

So now, somewhere in the later part of the 4th week, the heart is going to start to loop back on itself. It brings the apex of the heart to the left and the atrium and sinus venosus come to lie behind the truncus arteriosus, bulbus cordis and ventricle. That seems really complicated. Think of it more like this – which part was the blood coming out from? The truncus arteriosus, right? Which part of the adult heart does the blood come from? The ventricle, right? So we’re just curling that tube into itself to start to move things to the right places. It’s not that much more than when we discussed all the various folding and such that embryo does. Pretty much the same concepts, so don’t let yourself get overwhelmed.

Now that we have our cardiac loop, which is hanging “below” the foregut by the dorsal mesocardium, let’s start splitting up our chambers. These are also going to take place simultaneously, just like the folding did. The four partitions that happen are the atrioventricular canal, the atrium, the ventricle and the outflow tract.

Time reference, we’re about the end of week 4 here. Atrioventricular canal partitioning starts by the endocardial cushions appearing on the front and back walls of the AV canal. The next week, week 5, mesenchymal tissue, that all power pluripotent stuff, is going to invade the cushions. What does that do? Yup, they start growing quickly. Eventually, they’ll meet up and fuse, but leaving two passageways on either side so as to not cut off blood flow all together. Now we have our left and right AV canals! First one done!

Next one, the atrial partitioning – splitting it up into left and right again. Go to the roof of the primordial atrium and in the middle there will be a septum growing down headed towards those endocardial cushions we just talked about forming. Those basically formed the division of both atrium from the ventricles. This is splitting the atrium apart. So, that first septum is coming down and we call it the septum primum. It’s a pretty thin membrane that is kind of a crescent shape. It doesn’t full split the atrium apart, either. In fact, at the bottom is the foramen primum that is leaving an opening for blood below between the septum primum and the endocardial cushions. Before we fuse them together, apoptosis (programmed cell death) is going to start making holes towards the top of the septum primum. They’ll create the foramen secundum, still allowing blood to pass from right to left atrium.

After the septum primum has done its thing, the septum secundum grows next to it inside the right atrium. This is much thicker, but also crescent shaped. It keeps growign until it overlaps the septum primum sometime around week 5 or 6. So, now if you get past the septum secundum, you also have to get past the septum primum. But they’re leaving a little bit of a hole for blood to get through, yet. This is called for foramen ovale. Pretty soon, the cranial portion of the septum primum is going to disappear leaving sort of just the bottom flap. Basically, the septum primum is now acting like a valve so that blood can flow from the right atrium to the left but it will prevent back flow from left to right.

The atrial partitioning is a big thing to know. Failure in it leads to some pretty common congenital heart defects, meaning this is going to come up for you.

Another one that leads to some common, but less common, congenital defects is ventricular partitioning! That is not a very cheery segue, is it?

The septum in between the ventricles is made of both muscular and membranous parts. We call it the interventricular septum. Its growth starts around week 4 as myocytes from the primordial ventricle give rise to the muscular interventricular septum.

The way it works is the ventricles dilate and as it does so, the middle portion sort of folds in on itself, driving the spetum up interiorly. It doesn’t quite connect to the endocardial cushion quite yet. In fact, there is a foramen in there that allows blood to flow from the right and left ventricles. Next, we’re going to form the rest of the interventricular septum by finishing it off with the membranous portion. The endocardial cushions are going to work on merging with the left and right bulbar ridges and that will close the interventricular foramen.

Last, let’s make partition our outflow tract, still in that week 5 area. This time, we’re going to use neural crest mesenchyme in the bulbus cordis and the truncus arteriosus, which actually are going to make truncal ridges and those bulbar ridges that we just finished with in the ventricular partitioning. Remember, this is pretty much all happening about the same time. Oh, and those two sets of ridges are continuous with each other. So neural crest cells are coming from the primordial pharynx and the pharyngeal arches to reach these ridges. When they connect, its going to cause the bulbar and trunacl ridges to spiral 180 degrees. This is really important. Because as that spiral and the ridges form, we have sort of a big tube with the aorticopulmonary septum splitting it up. The two channels that are created are the ascending aorta and the pulmonary trunk.

Think for a moment, what if those didn’t twist? Can you see how big of an issue we would have if the ascending aorta and pulmonary trunk were reversed? It would mean that oxygenated blood would come from the lungs to the right atrium, down to the right ventricle and get pumped out back into THE PULMONARY TRUNK! Just making sure we’re clear that this seemingly minor detail is kind of really important.

Now that we’ve twisted and bent over the bulbus cordis, we’re going to fuse them up with the ventricles. On the right ventricle, we’re going to call it the conus arteriosus which is the very beginning of the pulmonary trunk. In the left ventricle, its the walls of the aortic vestiblue – the little hangy down bit below the aortic valve.

Which we need to put the valves in there, too. The semilunar cusps pretty much all just develop as swellings of subendoccardial tissue. They get hollowed out, hooked up with tissue whose muscles are dissolving (chordae tendinae) and those are connected to some strong parts of the trabeculae carnae (they’ll be the papillary muscles).

Okay, so we’re wrapping this up but we got the sinus venosus yet. It starts out as a separate chamber of the heart and it opens into the dorsal (back) wall of the right atrium. It has two horns coming off of it. The right horn, which is the part that opens into the right atrium, gets incoporated into the wall of the atrium and that is the sinus venarum. The left horn becomes the coronary sinus. the majority of the cardiac muscle’s blood supply ends up draining into the coronary sinus. Of course, the coronary sinus drains through the sinus venarum which also takes on all of the rest of the blood drainage. This stuff is smooth and we have some rough muscle in the atrium already, so now we got a mix of the two. They’re separated on the inside by a vertical ridge called the crista terminalis. On the outside of the heart, you can see a shallow groove that separates the two layers and this is called the sulcus terminalis.

And that is making the heart! Of course, it also needs to get hooked up to the pulmonary veins and it needs a conduction system. However, this post got plenty long the way it is. Hopefully, that cleared up some of the structural issues you might have been wondering about.